1. Field of the Invention
This invention is in the field of fiber lasers and amplifiers, and in particular, relates to the use multi-mode, rare-earth-doped, helical core fiber to construct high-power lasers and amplifiers.
2. Description of the Prior Art
Fiber lasers and amplifiers doped with rare-earth ions such as Yb, Nd, and Er have high potential for telecommunications, military, materials-processing, and other applications. In particular, the Yb-doped fiber laser with a single-mode core can be operated efficiently at high power to produce infrared light with near-diffraction-limited beam quality (V. Dominic, S. MacCormack, R. Waarts, S. Sanders, S. Bicknese, R. Dohle, E. Wolak, P. S. Yeh, and E. Zucker, 110 W fibre laser, Electron. Lett. 35, 1158-1160 (1999)).
Yb-doped fibers are pumped by radiation from arrays of diode lasers. Since the pump radiation has poor spatial beam quality, it is not possible to inject appreciable pump power into the fiber core. Instead, high-power operation has been achieved by injecting the pump light into the fiber cladding (U.S. Pat. No. 4,815,079). Since the core area is much smaller than the cladding area, pump absorption by the Yb typically occurs over tens of meters of fiber length. It is desirable to avoid a configuration where some of the pump light spirals around in the cladding without overlapping the core. Although displacing the core from the center of the fiber was proposed in U.S. Pat. No. 4,815,079 as a way to avoid such a situation, the commonly adopted technique is to inject the pump into an inner cladding with a noncircular cross section. Variations on such dual-clad fibers have been manufactured by companies including Polaroid, Lucent, Fibercore, and INO. Most fiber lasers manufactured using dual-clad fibers produce unpolarized light, especially in high-power operation.
In order to scale to higher power and to reduce the pump absorption length, it is desirable to use fibers with a larger core area. However, increasing the core area significantly normally results in multimode operation and hence loss of good beam quality. For step-index fibers, the parameter
V=(2xcfx80a/xcex)(n12xe2x88x92n22)1/2
determines whether the fiber supports only a single mode. Here a is the core radius, xcex is the wavelength, and n1 and n2 are the refractive indices of the core and cladding. Single-mode operation requires that V less than 2.405. Effectively single-mode operation of a fiber amplifier with V=7.4 has been achieved by coiling the fiber to induce significant bend loss for all but the lowest-order mode (J. P. Koplow, D. A. V. Kliner, and L. Goldberg, Single-mode operation of a coiled multimode fiber amplifier, Opt. Lett. 25, 442-444 (2000)). Unfortunately, coiling can also create unacceptable bend loss for the pump light.
Helical-core passive fibers were developed at the University of Southampton (U.S. Pat. No. 4,949,038). A preform with the core inserted off-axis was rotated during the drawing of the fiber. A fiber with V=25 at xcex=633 nm and a pitch varying continuously from 1.5 mm to several centimeters was manufactured and found to be effectively single-mode for a pitch between 9 and 1.8 mm. Moreover, helical fiber was found to exhibit strong circular birefringence. This implies states of circular polarization tend to be preserved during propagation through the fiber. Many applications, such as nonlinear frequency conversion and coherent beam combination, require a fixed polarization state at the output of the fiber.
A high power fiber laser/amplifier is comprised of a multi-mode rare-earth-doped helical-coiled core disposed within a cylindrical inner cladding. The inner cladding is enclosed within an outer cladding or thick polymer jacket to confine the pump radiation injected into the inner cladding. The core effectively produces single-mode operation with a circularly polarized near-diffraction-limited beam quality output for materials processing and other applications.